In a wireless receiver, the analog to digital converters (ADC) typically operate at low sampling rates. Hence, the received Radio Frequency (RF) signal must be mixed down to be processed. In a GSM receiver, the quadrature signals are down-converted to an Intermediate Frequency (IF) which can typically be either a zero intermediate frequency (ZIF) or very low intermediate frequency (VLIF).
In theory, the quadrature mixing provides infinite attenuation for the image band. However, in practice, there is always some imbalance between the in-phase (I) and quadrature phase (Q) branches, which can be due to amplitude and phase impairments between the local oscillator (LO) paths, mismatches between the respective I and Q branches after the analog down-conversion, as well as temperature variations and component ageing. This I/Q imbalance introduces unwanted spectral images which results in a mixture of the desired signals and interfering images.
When using a VLIF, signals on adjacent carriers can produce very strong interfering images of up to 50-100 dB stronger than the required signal (Valkama, M.; Renfors, M.; “Advanced DSP for I/Q imbalance compensation in a low-IF receiver,” IEEE International Conference on Communications, 2000. ICC 2000). However, using a non-zero IF can be beneficial in reducing the influence of flicker noise or 1/f noise. In order to avoid the problems caused by strong interfering images, the receiver requires either a very tight balance between the I and Q branches or some form of I/Q imbalance estimation and correction. Furthermore, it is usually beneficial to implement any I/Q imbalance estimation and correction scheme in the Digital Base-Band (DBB) of the wireless receiver as this will lead to a more power and area efficient solution than one implemented in the analogue domain.
Some of the existing I/Q imbalance estimation and correction solutions for low IF receivers use an adaptive approach as in the papers submitted by Valkama, M.; Renfors, M.; “Advanced DSP for I/Q imbalance compensation in a low-IF receiver,” IEEE International Conference on Communications, 2000. ICC 2000, and Elahi, I.; Muhammad, K.; Balsara, P. T.; “I/Q mismatch compensation using adaptive de-correlation in a low-IF receiver in 90-nm CMOS process,” IEEE Journal of Solid-State Circuits, February 2006. A low complexity solution is also presented by Mailand, M.; Richter, R.; and Jentschel; H.-J.: “I/Q-imbalance and its compensation for non-ideal analog receivers comprising frequency-selective components”, Adv. Radio Sci., 2006.
These existing solutions however do not utilize information on the characteristics of the signal being processed to improve the accuracy of the I/Q imbalance estimation and correction. Moreover, according to the preferred embodiments presented in this document, the I/Q imbalance estimation and I/Q imbalance correction are performed independently and such an approach provides further performance gains as well as enable the implementation complexity to be reduced.